Conventional resists comprising a novolak and a naphthoquinonediazide compound are unsuitable for use in pattern formation by lithography using far ultraviolet rays or excimer laser beams, because the novolak and the naphthoquinonediazide show intense absorption in the far ultraviolet region to render the light less apt to reach the resist bottom. Thus, the resist has low sensitivity to give only a tapered pattern.
One means for eliminating the above problem is the chemically amplified resist composition described in, e.g., U.S. Pat. No. 4,491,628 and European Patent 29,139. A chemically amplified positive resist composition is a pattern-forming material in which an acid generates in exposed areas upon irradiation with a radiation such as far ultraviolet rays and this acid catalyzes a reaction that makes the areas irradiated with the actinic rays and the unirradiated areas to differ in solubility in a developing solution to thereby form a pattern on a substrate.
Examples thereof include combinations of a compound which generates an acid upon photodecomposition with an acetal or O,N-acetal compound (see JP-A-48-89003; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an orthoester or amidoacetal compound (see JP-A-51-120714), with a polymer having acetal or ketal groups in the backbone (see JP-A-53-133429), with an enol ether compound (see JP-A-55-12995), with an N-acyliminocarbonic acid compound (see JP-A-55-126236), with a polymer having orthoester groups in the backbone (see JP-A-56-17345), with a tertiary alkyl ester compound (see JP-A-60-3625), with a silyl ester compound (see JP-A-60-10247), and with a silyl ether compound (see JP-A-60-37549 and JP-A-60-121446). These combinations show high photosensitivity since they have a quantum efficiency exceeding 1 as the principle.
Another means for eliminating the problem described hereinabove is a system which is stable over long at room temperature but decomposes upon heating in the presence of an acid to become alkali-soluble. Examples thereof include systems comprising a combination of a compound which generates an acid upon exposure to light with an ester having a tertiary or secondary carbon (e.g., t-butyl or 2-cyclohexenyl) or with a carbonic ester compound, as described in, e.g., JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642; Polym. Eng. Sce., Vol. 23, p. 12 (1983); ACS. Sym., Vol. 242, p. 11 (1984); Semiconductor World, November 1987 issue, p. 91; Macromolecules, Vol. 21, p. 1475 (1988); and SPIE, Vol. 920, p. 42 (1988). Since these systems also have high sensitivity and show reduced absorption in the deep UV region as compared with the naphthoquinonediazide/novolak resin systems, they can be effective systems for coping with the utilization of the light source having shorter wavelength.
The chemically amplified positive resist compositions described above are roughly divided into three groups: three-component systems comprising an alkali-soluble resin, a compound which generates an acid upon exposure to a radiation (photo-acid generator), and a dissolution inhibitive compound for the alkali-soluble resin which has acid-decomposable groups; two-component systems comprising a resin having groups which decompose upon reaction with an acid to render the resin alkali-soluble and a photo-acid generator; and hybrid systems comprising a resin having groups which decompose upon reaction with an acid to render the resin alkali-soluble, a low-molecular dissolution inhibitive compound having an acid-decomposable group, and a photo-acid generator.
In JP-A-2-19847 is disclosed a resist composition characterized by containing a resin obtained from poly(p-hydroxystyrene) by protecting all or part of the phenolic hydroxyl groups each with a tetrahydropyranyl group.
In JP-A-4-219757 is disclosed a resist composition characterized by containing a resin obtained likewise from poly(p-hydroxystyrene) by replacing from 20 to 70% of the phenolic hydroxyl groups each with an acetal group.
Moreover, JP-A-5-249682 discloses a photoresist composition containing a similar resin protected with an acetal. In JP-A-8-123032 is disclosed a photoresist composition containing a terpolymer having groups substituted with acetal groups.
Furthermore, JP-A-8-253534 discloses a photoresist composition containing a partly crosslinked polymer having groups substituted with acetal groups.
However, these prior art compositions have a problem that they are unsuitable for use in producing IC elements on the order of sub-half-micron since resist patterns formed therefrom have depressions in an upper part thereof. Those compositions have another problem that the resist patterns have poor adhesion to substrates and this arouses troubles such as fine-pattern collapse.
As described above, the prior art photoresist compositions have problems that they are unsuitable for use in producing IC elements on the order of sub-half-micron since resist patterns formed therefrom have depressions in an upper part thereof, and that the pattern/substrate adhesion is poor to arouse troubles such as fine-pattern collapse. Hence, further improvements have been desired.